Individual coupling caps for each power tube?

[Gaz]

In a few four+ power tube amps I've noticed that each tube has its own coupling cap and grid leak resistor. This is in contrast to most poweramps that use only one coupling cap/grid leak per side of the push-pull.

I was just curious what advantages there would be for each power tube to have its own cap?

[diagrammatiks]

you need it if you have individual bias adjustment for each output tube.

[martin manning]

It is required for individual bias adjustment, and even if that is not the case, it isolates the tubes during transients. As one tube's grid is driven positive with respect to it's cathode, it begins to draw grid current and experiences bias excursion. With this isolation, the effect on the bias condition of the adjacent tube(s) will be limited.

[Structo]

Interesting.

On my 100w ODS clone, I have dual bias pots, one for each side.

Does my amp need individual caps?

[ChrisM]

Interesting.

On my 100w ODS clone, I have dual bias pots, one for each side.

Does my amp need individual caps?

It already does.

[Structo]

Well I have one coupling cap for each pair of tubes.

[martin manning]

A cap for each tube is only necessary if you want a separate bias pot for each tube.

[Structo]

OK thanks, my brain isn't working too good lately.

[Gaz]

Thanks for the replies. I understand the part about separate bias adjustments, but none of the amps I was referring to have separate adjustments. I should've been more specific, but I was thinking of The Hiwatt DR201 and Carlsbro TC200 as well as the 200 watt Sound city amps.

Because this only seems to be the norm on bigger amps (not the Marshall Major, however), I think Martain is probably right that it has something to do with limiting bias excursion. Still wonder how much it'd make a difference.

[diagrammatiks]

It has to do with loading as well.

If you look at the 400watter, they have a driver tube and only 1 set of coupling caps.

[Gaz]

It has to do with loading as well.

If you look at the 400watter, they have a driver tube and only 1 set of coupling caps.

I'm not sure I know what you mean.

[David Root]

Correct me if I'm wrong, but if you use four coupling caps instead of two, shouldn't they each be half the value of each of the two caps? (Capacitance in parallel adds). So that the pair's time constant doesn't change?

[Firestorm]

Correct me if I'm wrong, but if you use four coupling caps instead of two, shouldn't they each be half the value of each of the two caps? (Capacitance in parallel adds). So that the pair's time constant doesn't change?

I think you also double the grid load to perfectly replicate the RC time constant.

[David Root]

I think that's right. 220K to two tubes is effectively 440K to each one.
Shows how conservative the Ancients were at 100K per tube!

I guess I was thinking as much about making the amp too bassy if you don't cut the caps' values in half.

[Firestorm]

I think that's right. 220K to two tubes is effectively 440K to each one.
Shows how conservative the Ancients were at 100K per tube!

I guess I was thinking as much about making the amp too bassy if you don't cut the caps' values in half.

Some tubes, though, are quite fussy about grid circuit resistance. 6550s, in particular, are easy to kill with too much.